The coating of electrically conductive substrates by electrodeposition is a well known and important industrial process. (For instance, electrodeposition is widely used in the automotive industry to apply primers to automotive substrates). In this process, a conductive article is immersed as one electrode in a coating composition made from an aqueous emulsion of film-forming polymer. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is produced on the article. The article to be coated is the cathode in the electrical circuit with the counter-electrode being the anode.
Resin compositions used in cathodic electrodeposition baths are also well known in the art. These resins are typically manufactured from polyepoxide resins which have been chain extended and adducted to include a nitrogen. The nitrogen is typically introduced through reaction with an amine compound. Typically these resins are blended with a crosslinking agent and then neutralized with an acid.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives (usually at the coating site) to form the electrodeposition bath. The electrodeposition bath is placed in an insulated tank containing the anode. The article to be coated is made the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and so forth.
The coated object is removed from the bath after a set amount of time. The object is rinsed with deionized water and the coating is cured typically in an oven at sufficient temperature to produce crosslinking. It is then typically covered with any of a variety of different topcoat systems (e.g. basecoat/clearcoat).
The prior art of cathodic electrodepositable resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in U.S. Pat. Nos. 3,922,253; 4,419,467; 4,137,140; and 4,468,307.
Additives are a very important part of an electrocoat primer composition. Numerous different types of additives can be added. For instance film build additives are sometimes used to increase film build. High film build is important to maintain equivalent corrosion resistance without the use of a primer surfacer over the electrocoat. Generally solvents are also needed to maintain film build in the electrocoat system when using additives. Unfortunately the addition of solvents can result in higher than satisfactory volatile organic content (VOC). Another potential problem with many additives is that they can retard curing which can have an adverse effect on chip resistance (i.e. the adhesion of the electrocoat primer to the substrate) and intercoat adhesion (i.e. the adhesion of a topcoat to the electrocoat primer) This detrimental effect on the curing (and resultant poor chip resistance and intercoat adhesion) is especially pronounced at underbake conditions. Underbake is the lower limit of the cure temperature range of a given system. Underbake in current commerical cathodic electrocoat systems is about 20.degree. F. to 30.degree. F. lower than the standard bake temperature, for the same bake time. Underbake performance is important because some parts of the substrate being baked may not reach the optimum bake temperature.
What is needed is an additive which will promote chip resistance and intercoat adhesion, help lower VOC and improve film build.